1. Field of the Invention
This invention relates to an improved, internal-diameter flail tube cutter which utilizes a resilient bladder to temporarily affix the cutter in a desired position along the longitudinal axis of a tube. It is particularly useful in cutting sample heat exchange tubes from a nuclear steam generator.
2. Description of the Prior Art
Internal-diameter tube cutters are generally known in the prior art. One of the most common types of such cutters includes a pair of opposing blades which are outwardly extendable by means of a cam. In operation, this type of a cutter is slid into the tube to be cut, and the blade-extending cam is forcibly wedged through cam slots in the blades while the cutter is rotated. The cutting action of such cutters is similar to that of an ordinary household can opener, wherein the workpiece is rotated relative to the blade while a steady pressure causes the blade to cut into the wall of the tube.
While such internal-diameter tube cutters provide satisfactory results in many applications, certain problems may arise when tube cutters of this design are used to cut one or more sample heat exchange tubes out of a nuclear steam generator, which is sometimes necessary in order to determine the extent to which the heat exchange tubes are experiencing corrosion. For example, if the internal diameter of the heat exchange tube to be sampled has been dented around the area of the generator support plates, or internally sleeved to prevent a leak condition from arising in the tube, it may not be possible to slide a conventional, internal-diameter tube cutter through the restrictions in the tube caused by either such dents or sleeves. If one attempts to solve this problem by merely reducing the width of such cam-operated tube cutters so they can slide through such restrictions, the mechanical strength of the cutter may be diminished to the point where it breaks down or wears out after a few tube cuts.
Still another problem associated with such cam-operated tube cutters is the relatively long period of time it takes to penetrate the walls of the tube with such a cutter, coupled with the incompatibility with presently known robotic manipulators. This is a particularly acute problem when tube samples are being cut from an on-line nuclear steam generator, where the longer such a tube cutting process requires, the more radiation the maintenance personnel performing such cutting processes are exposed to.
To solve the problems associated with such prior art tube cutters, a flail tube cutter was developed by Mr. Edward Chobey of the Westinghouse Electric Corporation. This flail tube cutter is described and claimed in U.S. patent application Ser. No. 631,371 filed July 16, 1984, the entire specification of which is expressly incorporated herein by reference. This flail tube cutter is generally comprised of an elongated, substantially cylindrical cutting head which is circumscribed at a point near its top end by a shallow, tapered cutting blade. This cutting blade is serrated at one point in order to define a pair of cutting teeth. The bottom end of the cutting head is connected to a flexible, high-speed drive shaft. Like a train with a partially flattened wheel, the sharp edges defined by the serration prevent any opportunity for a smooth, wheel-to-wheel engagement between the circular cutting blade and the inside wall of the tube. The end result is that the high-speed, flexible shaft whips and flails the cutting head and its shallow, serrated blade against the inner walls of the tube with sufficient force to create a series of overlapping nicks which eventually become overlapping perforations as the tube is finally cut. The cutting head and blade can be easily fabricated with a sufficiently small diameter so that the cutting head and its flexible drive shaft are easily inserted into the open end of a tube and snaked to any desired position along its longitudinal axis.
Unfortunately, this flail tube cutter is not without certain limitations. While it does provide a small-diameter cutting head which is capable of cutting completely through the walls of an Inconel.RTM. heat exchange tube in a matter of a few seconds, the squirming of the high-speed, flexible shaft sometimes causes the serrated, circular blade to perforate the tube in a pattern resembling a broad ring, rather than a thin circle. This ring-shaped cutting pattern becomes more pronounced the farther the cutting head and flexible shaft are snaked up the tube toward the top, U-bend portion thereof, since the squirming of the flexible drive shaft worsens with length. The end result is that this flail tube cutter will sometimes produce a jagged cut around the tubve. A neat well-focused cutting pattern is more desirable than a jagged cut, because it minimizes the amount of metallic debris associated with the cut, and renders it easier to withdraw the sample tube through the tubesheet for inspection.
Still another limitation of this prior art flail tube cutter is the fact that it is not designed to be inserted into the open ends of the tubes from the primary side of the generator, where most tube maintenance procedures are carried out. Rather, it is designed to be inserted through an open end of a tube from the secondary side of the generator, after a U-bend section of a tube has been cut out by an external tube cutting tool. Finally, like the cam-type cutter that preceded it, this prior art flail tube cutter is not compatible with known robotic manipulating devices, thereby necessitating the use of a human operator in a radioactive environment.
Clearly, there is a need for an internal diameter tube cutting device having a diameter which is small enough to slide around local obstructions in the tubes, but is capable of quickly, accurately and neatly cutting the tubes at any position along their longitudinal axis. Ideally, such a tube cutter should be usable from both the primary and secondary sides of the generator, simple in construction, positionable within curved as well as straight tubes by means of known robotic manipulators, and capable of remotely cutting these tubes with a minimum amount of operator effort. Finally, it would be desirable if such a cutter were usable in tubes having different diameters, and included some sort of means for quickly and easily changing the blades as they wore out.